1. Field of the Invention
The present invention relates to an optical pickup apparatus and a focal-point adjusting method, and more particularly, relates to an optical pickup apparatus and focal-point adjusting method suitable in use at the time of recording to and reproducing from a recording medium stacked thereon with a plurality of recording layers.
2. Description of the Related Art
In the recent years, along with the increased capacity of optical discs, the multilayered recording layers have been advanced. By including a plurality of recording layers in a single disc, the data capacity of the disc can be increased remarkably. When stacking the recording layers, the general practice until now has been to stack two layers on one side, but recently, to further advance the large capacity, disposing three or more recording layers on one side is also examined. Herein, when the number of recording layers to be stacked is increased, the large capacity of a disc can be promoted. However, on the other hand, the space between recording layers is narrowed, and signal degradation caused by crosstalk between layers increases.
If the recording layer is multilayered, a reflected light from the recording layer to be recorded or reproduced (target recording layer) becomes very weak. Therefore, when unnecessary reflected light (stray light) enters a photodetector from the recording layers present above and below the target recording layer, the detection signal is degraded, which may exert an adverse effect on a focus servo and a tracking servo. Therefore, when a large number of recording layers are disposed in this way, the stray light needs to be removed properly so as to stabilize the signals from the photodetector.
Herein, a method for removing the stray light includes that which uses a pinhole. In this method, a pinhole is disposed at a convergence position of the signal light. According to this method, a part of the stray light is intercepted by the pinhole, and therefore, the unnecessary stray light component entering the photodetector can be reduced. Another method for removing the stray light includes that which combines ½ wavelength plates and polarized light optical elements. According to this method, a polarization direction of the stray light is changed by the ½ wavelength plates, and the stray light is intercepted by the polarized light optical elements. Thus, the unnecessary stray light component entering the photodetector can be removed.
However, in the case of the method for removing the stray light by using a pinhole, the pinhole needs to be positioned accurately at the convergence position of a laser light (signal light) reflected from the target recording layer, and therefore, a task for adjusting the position of the pinhole is difficult, thus posing a problem. If the size of the pinhole is increased to facilitate the task for adjusting the position, the proportion of the stray light passing through the pinhole increases, and the signal degradation caused by the stray light cannot be inhibited effectively.
Furthermore, in the case of the method in which the ½ wavelength plates and the polarized light optical elements are combined to remove the stray light, apart from the fact that the ½ wavelength plates and the polarized light optical elements two each are needed to remove the stray light, a user needs to have two lenses, which increases the number of components and the cost, and adjusting the placement of each component is a complex process, thus posing a problem. Furthermore, the user needs to have a space for placing and arraying these components, which results in the enlargement of the optical system, thus posing a problem.